It has long been known that the coating of a number of different photographic emulsions on a film, can be carried out particularly economically if all the emulsions can be applied simultaneously in one operation. A method which has become known as the cascade pouring process is carried out by means of an apparatus consisting of a number of blocks assembled to form a sloping upper surface and a body containing a longitudinal distribution duct in each block, an individual coating liquid being fed separately to each duct. The number of blocks corresponds to the number of liquids to be poured. A substantially vertical slot is provided with which each distribution duct communicates and in these slots the liquids are fed upwards by pressure from the distribution duct. At the top end the slots all open into the inclined and substantially plane surface on which the liquid flows down as a coherent uniformly distributed coating. The planes associated with the individual slots may be coplanar or be offset from one another in the region of the individual slots by small steps corresponding substantially to the thickness of the coating. In these conditions, the liquids whose exit slots are situated in the lower region of the outlet plane have the liquids emerging farther above superimposed on them with a laminar flow so that at the bottom end of the outlet surface the liquid film finally formed is built up from a plurality of sharply separated layers. This liquid film is applied either as a freely suspended meniscus or bead, or as a free-falling curtain on to the moving surface for coating. In the former case the surface for coating is spaced at a small distance from the bottom end of the outlet surface, said distance usually being only fractions of a millimetre; in the latter case, the distance is such that a free-falling curtain can form. With this apparatus it is possible without difficulty to apply three, four or even more coatings simultaneously to the surface for coating and experience has shown that if the process is properly performed the individual coatings will not mix with one another when they flow down over the inclined plane, in the freely suspended meniscus or the free-falling curtain, and on the moving surface during the subsequent drying operation.
Those versed in the art, however, are aware of the fact that the cascade pourer has a number of serious disadvantages, which become more troublesome the greater the number of superimposed liquid layers requiring to be applied in a single operation. For example, the liquid film, consisting of a plurality of layers and running over the inclined plane of the cascade pourer by gravity, is exposed to certain spontaneous disturbances which depend, in a complex manner which is not fully understood, upon the number of layers of liquid and the relationship between their thickness and viscosity. Such disturbances take the form of, for example, spontaneous corrugations perpendicularly to the direction of flow, and when the coating is applied to the base or support such corrugations remain in the form of periodic variations in emulsion thickness and have a very adverse effect on the uniformity of the coating. The probability of such corrugations occurring increases with the distance that the free-flowing film has to cover and with the thickness of the layer of liquid. For this reason, conditions become increasingly unfavourable for each additional coating.
Another very serious type of disturbance is produced by the longitudinal lines ("pencil lines") which occur in the individual emulsions and which may be due, for example, to discrete particles or air bells lodging in the vertical distribution slot associated with each coating liquid, its top edge or the outlet surface.
The strictly laminar flow pattern means that such disturbances are maintained for a long period. The transverse or longitudinal lines may even out by flow of liquid transversely thereto, but this can be expected only if the disturbed coating has a surface which is exposed to the atmosphere. In such cases the high surface tension of the liquid produces at least a partial compensation as a result of the elastic properties of the liquid surface. However, in the case of the cascade pourer, only the liquid emerging from the top slot has a free surface exposed to atmosphere. All the other coatings have the layers of liquid farther above superimposed on them immediately they emerge from the distribution slot. Although there are generally interface tensions between the individual layers of liquid, they are several orders of magnitude less than the tensions existing with respect to atmosphere and they correspond only approximately to the difference between the normal surface tensions. Consequently, all the disturbances occurring in any layer which is not situated at the surface will not even out significantly within the time available for the coating process, but will be transmitted to the support or base in their original extent. This tendency is further assisted by the fact that the differences in density of adjacent layers of liquid are generally very small so that gravity which would assist evening out is restricted to a minor value.
Calculation and experiment clearly show that any disturbance occurring in a bottom layer not immediately adjacent the free surface affects not only said layer but also the adjacent layers in sympathy. It will readily be seen that the probability for the occurrence of such faults increases in proportion with the number of layers poured simultaneously.
Another disadvantage of the prior-art cascade pourers is due to their construction. For practical reasons, the pouring apparatus is always constructed from an individual block for each layer, in which the recesses for the liquid feed, distribution ducts and exit slots are formed. These blocks are assembled by means of devices of various design, to form the complete pourer head and are retained in position by suitable supports. The strict requirements applicable to the production of photographic materials means that the construction and assembly of the pourer head must be carried out with the utmost precision. More particularly, the distribution slots must be machined very accurately since any variation in their width is cubed in its effect on the flow of the corresponding liquid. With the standard cascade pourer construction it is not possible to correct an individual slot by mechanical clamping or bending as is conventional practice with extruders, for example, because any deformation of an individual block influences the distribution slots situated on either side of it. This disadvantage of the cascade pourer also has increasing effect in proportion to the number of coatings to be poured from a single pourer head, and hence also the number of assembled blocks. Mechanical stresses which may occur if any of the blocks undergo uneven heating, also have an unfavourable effect in the same way.
Finally, the fact that the individual blocks from which a cascade pourer is made must have a minimum thickness, for mechanical reasons, also has an adverse effect. For example, after the distribution duct has been cut, a minimum material thickness must always be left to give the plate the necessary strength to resist mechanical and thermal distortion. It will readily be seen that any cascade pourer designed for the simultaneous pouring of three or more emulsions consequently has a very considerable weight, so that assembly, handling and cleaning are made difficult. At the same time, sensitivity to mechanical thermal distortion is increased disproportionately with each additional distribution plate.
The object of the invention is to provide coating apparatus in which the above disadvantages may be at least reduced.